Three-dimensional covalent organic frameworks with pto and mhq-z topologies based on Tri- and tetratopic linkers

Three-dimensional (3D) covalent organic frameworks (COFs) possess higher surface areas, more abundant pore channels, and lower density compared to their two-dimensional counterparts which makes the development of 3D COFs interesting from a fundamental and practical point of view. However, the construction of highly crystalline 3D COF remains challenging. At the same time, the choice of topologies in 3D COFs is limited by the crystallization problem, the lack of availability of suitable building blocks with appropriate reactivity and symmetries, and the difficulties in crystalline structure determination. Herein, we report two highly crystalline 3D COFs with pto and mhq-z topologies designed by rationally selecting rectangular-planar and trigonal-planar building blocks with appropriate conformational strains. The pto 3D COFs show a large pore size of 46 Å with an extremely low calculated density. The mhq-z net topology is solely constructed from totally face-enclosed organic polyhedra displaying a precise uniform micropore size of 1.0 nm. The 3D COFs show a high CO2 adsorption capacity at room temperature and can potentially serve as promising carbon capture adsorbents. This work expands the choice of accessible 3D COF topologies, enriching the structural versatility of COFs.


Transmission electron microscopy (TEM): TEM was performed on a Titan Themis Scan/transmission
electron microscope operated at 300 kV. Powder samples were briefly sonicated in ethanol before being dropped on the grids. Thermogravimetric analysis (TGA): TGA measurements were performed on Q-600 Simultaneous TGA/DSC from TA Instruments under argon atmosphere by heating to 800 °C at a rate of 10 °C min -1 .
Solution nuclear magnetic resonance (NMR): Solid state 13 C cross-polarization magic angle spinning NMR spectra were obtained on a Bruker 500 MHz spectrometer operated at room temperature. spectrometer using a double-resonance magic-angle spinning (MAS) probe head for 4-mm rotors. The 13 C cross-polarization (CP) spectra for COFs RICE-3/4/5/6/7 were obtained with 7.6 kHz MAS, 2.5 ms contact time, and a 5 s relaxation delay. To distinguish between protonated and non-pronated carbon atoms, 13 C CP spectra with non-quaternary suppression (CPNQS) with 50 µs and 80 µs dephasing delays were also acquired. For each 13C CP /CPNQS experiment, five to six thousand scans were accumulated. All the spectra were processed with 50 Hz line broadening. Chemical shifts are relative to glycine carbonyl defined as 176.46 ppm.  4 in the projector augmented wave (PAW) 5 formalism were used to treat the nuclei and frozen-core electrons. Explicit and self-consistent calculations were S4 performed to treat the following valence electrons: N-2s 2 2p 3 , O-2s 2 2p 4 , H-1s 1 , and C-2s 2 2p 2 . The kinetic energy cutoff was set at 450 eV to truncate the plane-wave basis sets. Grimme's D3 dispersion 6 was employed to describe the van der Waals interactions. We also tested the dispersion correction term using DFT-D2 approach of Grimme 7,8 and DFT-D3 method with Becke-Johnson damping function 6,9 and found the different methods generate a difference within ±0.2 eV, which justifies our choice of DFT-D3 for dealing with van der Waals interaction. All calculations were spin-polarized. The smearing width of the Gaussian smearing scheme was 0.05 eV. A Monkhorst-Pack (MP) 10 k-point mesh of 1×1×1 was sampled on the unit cell. The bulk structure of COF RICE-7 was first obtained from powder X-ray diffraction (PXRD) measurements. DFT was then applied to optimize the structure with a force convergence criterion of 0.02 eV Å −1 and a selfconsistent-field electronic energy convergence criteria of 10 −5 eV. The optimum lattice constants were (32.57 Å, 32.57 Å, 32.57 Å) and (60°, 60°, 60°), determined from cell relaxations in which the volume of the unit cell was allowed to change. The adsorption energy for each structure was estimated by taking the total DFT energy of the adsorption configuration and subtracting the total energy of the reference structure (i.e., the structure with the CO2 molecule in the center of the largest pore shown in Supplementary Fig. 81). The tube was further sealed, placed in oven and heated under 120 °C for 3 days. All of the products were separated and washed thoroughly using THF and ethanol. The wet powder samples were sealed in a tea bag and dried using the Leica EM CPD300 Critical Point Dryer.
We also tried to use a solvent mixture of o-dichlorobenzene (o-DCB)/1-butanol (1-BuOH)/ 6M acetic acid (v/v/v, 5/5/1) at the same reaction conditions (120℃, 7 days), however, we could not produce highly crystalline COFs. We also tried to use a solvent mixture of dioxane/mesitylene/ 6M acetic acid (v/v/v, 4/1/1) under the same reaction conditions (120℃, 7 days), however, the yield of COF powders was very low (less than 3%). All the products were separated and washed thoroughly using THF and ethanol. The wet powder samples were sealed in a tea bag and dried using the Leica EM CPD300 Critical Point Dryer.
We also tried to use solvent mixture of dioxane/mesitylene/ 6M acetic acid (v/v/v, 4/1/1) under the same reaction conditions (120℃, 7 days), however, this condition could only produce amorphous powders. S8 0.5 mL 1-butanol (1-BuOH) in a Pyrex tube directly without degassing. Afterwards, 0.2 mL 6M acetic acid was added and the solution was sonicated for 10 minutes. The tube was further sealed, placed in oven and heated under 120 °C for 7 days. All of the products were separated and washed thoroughly using THF and ethanol. The wet powder samples were sealed in a tea bag and dried using the Leica EM CPD300 Critical Point Dryer.

Digestion of COF RICE-3 and 1 H NMR test
To probe the monomer ratios in COF-RICE-3, dry COF powders were digested in a glass vial using a solvent mixture of DMSO-d6 and deuterium chloride solution (35 wt. % in D2O, ≥99 atom % D) (v/v, 10/1).
To note, we tried multiple solvent conditions to digest the COF powders. RICE-3 could not be digested using solvent mixtures including trifluoroacetic acid/water and formic acid/water even at 80 ℃. RICE-3 could be digested using DMSO and NaOH at 60 ℃ and could not be digested at room temperature. However, NaOH precipitated out at room temperature, and it was not convenient, so we chose the solvent mixture of Supplementary Fig. 75. 1

Digestion of COF RICE-7 and 1 H NMR test
To probe the monomer ratios in COF-RICE-7, dry COF powders were digested in a glass vial using 1 mL DMSO-d6, 0.1 mL deuterium oxide (99.9 atom % D), and 40 mg NaOH solids. The vial was sonicated and then heated on a hot plate at 80 ℃ until the solution was clear. Afterward, the clear solution was transferred into an NMR tube and tested for 1 H NMR. To prevent the NaOH solids precipitate out, the NMR tube was kept warm using a hot cotton cloth before the test. To note, we also tried multiple solvent mixtures to digest RICE-7. The digestion of RICE-7 was more challenging than that of RICE-3. RICE-7 could not be digested using formic acid/water even at 80 ℃, where the solution was cloudy and suspended sheets existed. In the solvent mixtures of trifluoroacetic acid and water with water contents ranging from 5 v% to 20 v%, RICE-7 could not be digested even at 80 ℃, and many particles existed. Similarly, RICE-7 could not be digested using DMSO/35 wt% HCl both at room temperature and 80 ℃, and particles existed in the solution. The combination of DMSO and NaOH was the only solvent found that could digest RICE-7 at 80 ℃.
The result was shown in Supplementary Fig. 76 and the peaks were indexed (unassigned peaks may come from oligomers and Cannizzaro reaction might have occurred in the base conditions). We use peak "a" as a standard (assume we have 1 mol amine), so the integration of a is 8 mol, the theoretical integration of c is 2 mol, and the integration of 1+2+3 is 11.28+10.83-2=20.11 mol. The ratio of amine and aldehyde is 1: (20.11/(6+6+3))= 1:1.34, very close to 3:4.
Supplementary Fig. 76. 1 Figs. 80 -82). All the COFs studied showcased high chemical stability as indicated by the consistent PXRD and FTIR measurements.
We also analyzed the pore collapse stability of RICE-3 and RICE-4 to see if the methyl groups on the side functionalities have significant influence on their pore collapse stability. Supplementary Fig. 83 indicates that RICE-3 and RICE-4 are stable upon activation with hexane and ethanol, which have lower surface tension solvents, and they collapse after activation with THF which has high surface tension. This DFT calculation of CO2 adsorption. We studied CO2 adsorption in the COF RICE-7 structure using density functional theory (DFT). Several candidate adsorption sites were evaluated including both chemisorption and physisorption mechanisms. Within our trials, the chemical bonds between the chemisorbed CO2 and RICE-7 always broke as the geometry optimization progressed, resulting in the formation of the linear CO2 molecule. This indicates that CO2 chemisorption via the formation of chemical bonds with the COF framework is not feasible. Thus, CO2 adsorption in COF occurs through physisorption mediated by van der Waals interactions. We set the energy of the CO2 molecule positioned at the center of the largest hole in RICE-7 as the reference state for computed adsorption energies, as the CO2 molecule in the position does not interact with the COF framework (i.e., because the two are separated by at least13 Å) (Supplementary Fig. 85a). When CO2 is put in a smaller pore in closer proximity to the conjugated COF structure, then the computed adsorption energy is -0.12 eV, as shown in Supplementary Fig. 85b. Thus, CO2 can be stabilized by van der Waals interactions within the COF material. Moreover, when the orientation of the C=O bond of the CO2 molecule is parallel to the C=C bonds and C=N bonds in the COF structure ( Supplementary Fig. 85c), then the adsorption is even more favorable (-0.45 eV). This is attributed to the interaction between the CO2 molecule and the electron-rich, conjugated p orbitals of the COF framework. This adsorption strength is comparable to CO2 adsorption in zeolites 23 and Metal-Organic Frameworks (MOFs) 24,25 , which are known CO2 absorbents. These calculations show that RICE-7 is a suitable material for CO2 capture applications.